Please help me clarify overall focus. I have found snippets of information, but nothing to pull it all together.

I understand item focus. If you are wearing a specific level (max) focus and cast a spell higher than that, you still get the benefit of focus, just reduced for every level above the level of the focus.

I understand tribute focus. Basically, the same as item focus. Do they stack?

I think I understand potion focus. I say "I think" because I have seen comments where it is noted that a lower level potion still gives benefits to "higher" level spells (i.e. HoTs). Is this because having the buff automatically adds > 0% benefit which automatically is "rounded" to 1 additional tick? So, is it just as effective (and cost efficient) to carry the VII version rather than having the X version around? Someone asked the question in another thread about potion level VII still giving an extra HoT tick at 95. Do they stack? Alla's info included for reference:

The big question is: Do all these effects stack? By running a potion and tribute while wearing the focus on items, do I get benefit from all? Is there a cap? Do I need to go to max tribute (X) and max potion (X) or can I get by with a lower level tribute or potion focus level? Presumably, this is the same for all focusable (is that a word) spells (DoTs, not just HoTs).

Does this mean that since they are both in slot 1, they are not additive with each other even though the item does not present a spell buff icon (and therefore, the item increase of 26% negates the potion buff)? Or, since the item does not present a spell buff icon, the spell buff icon isn't blocked and they are, in fact, additive. Thus, with grasp of the beast, you get an additional 26% duration out of every detrimental spell up to level 95 that is at least 24.00 sec long (not including complete heal, invulnerability, reaction radius or combat skills) and an additional 5% on top of that, if you have the potion buff on? Is that simple math 31% extension or an additional 5% onto the 26% (another 1.4%)?